Charge density wave and superconductivity in 2H- and 4H-NbSe2: A revisit

Abstract

Good-quality hexagonal NbSe2 single crystals were prepared. In 2H-NbSe2, superconducting and charge density wave (CDW) transitions were found at T s = 7.4 K and T c = 35 K respectively as reported previously. We have noticed that these two transitions are changed to T c = 42 K and T s = 6.5 K, in 4H-NbSe2. Thermopower has shown clear anomaly at CDW transitions. The anisotropic upper critical field was calculated as ~3 and 6.3 for 2H- and 4H-single crystals around t = 0.81, where t = T/T s, from resistivity and explained in terms of coherence length. From the relation, $H_{\rm c2} (T)=H_{\rm c2} (0)[1-t^2]$ , $H_{\rm c2}^l (0)$ was calculated as ~8.15 T and 16.98 T at t = 0.84 in 2H-NbSe2 and 4H-NbSe2 respectively. However, $H_{\rm c2}^t (0) = 2.68$ T for both single crystals.